1. Field of the Invention
The present invention relates to an image coding apparatus, an image coding method, an image decoding apparatus, an image decoding method and a transmission medium, applicable for a system which codes or decodes an image at a high efficiency and transmits or efficiently stores the image. More particularly, the invention relates to an image coding apparatus, an image coding method, an image decoding apparatus, an image decoding method and a transmission medium which are applicable to a compressing and extending apparatus of a high-precision fine image such as a satellite image or a medical image or a software module thereof, or a compressing and extending apparatus of texture used in games or three-dimensional CG or a software module thereof.
2. Description of the Related Art
One of the typical conventional image compressing methods is the JPEG (Joint Photographic Experts Group) method standardized by the ISO (International Organization for Standardization). This is an image compressing method using DCT (Discrete Cosine transform), in which a satisfactory coded or decoded image is available when it is possible to assign relatively numerous coding bits to each pixel.
If the number of coding bits is reduced to below a certain level, however, there occurs serious block distortion unique to DCT, leading to apparent deterioration of image subjectively. Various organizations have therefore proposed techniques for solving this block distortion unique to DCT, including, for example, a technique using DC/AC conversion known as MDCT (Modified DCT) is disclosed by J. Princen, et al. in xe2x80x9cSubband/Transform Coding Using Filter Bank Designs Based on Time Domain Aliasing Cancellationxe2x80x9d released in the IEEE Proceedings ICASSP 87, 50.1, pp. 216102164, 1987. According to this MDCT, block distortion becoming apparent in DVT is known to be reduce by performing transformation while overlapping a block with the surrounding blocks. In general, MDCT can transform 2M time-serial sample data into M spectra, and perfectly restore the original: time-serial sample data by means of IMDCT (inverse MDCT).
As conventional arts concerning MDCT, these are available a xe2x80x9csignal transforming apparatusxe2x80x9d disclosed in Japanese Patent Publications Nos. 6-66067 and 06-66068, and xe2x80x9ca normal transform calculating apparatus and an inverse transform calculating apparatus for improved DCTxe2x80x9d disclosed in Japanese Unexamined Patent Publication No. 4-44099. These inventions relate to a method for reducing the hardware volume of MDCT and a high-speed calculation of MDCT.
In an xe2x80x9cimage coding apparatusxe2x80x9d disclosed in Japanese Unexamined Patent Publication No. 7-50835, on the other hand, it is taught that a dynamic image can be coded by splitting the image into subbands, and applying quadrature transformation to the thus split low-level components.
Unlike the reduction of the hardware volume of MDCT or achievement of a higher speed of calculation, the objects of the Japanese patent Publications Nos. 6-766067 and 6-66068 and Japanese Unexamined Patent Publication No. 4-44099, the present invention has an object to accomplish high-efficiency image coding and decoding by the concrete application of MDCT as a technique for quadrature transformation. Therefore, Japanese Unexamined Patent Publication No. 7-50835 in which quadrature transformation is actually applied and subband splitting is carried out in the first stage of the process will be described as an example of the conventional arts.
FIG. 18 is a block diagram illustrating a configuration of a first embodiment of the aforementioned Japanese Unexamined Patent Publication No. 7-50835.
In FIG. 18, a subband splitter 200 can subband-split an original image into a low-level image and a plurality of high-level images. A quadrature transformer 201 applies quadrature transformation by splitting a low-level image 301 provided as an output from the subband splitter 200 into blocks of a first size, and outputs the same as blocks 302. A block splitter 202 splits a plurality of high-level images generated by the subband splitter 200 and supplied via the quadrature transformer 201 into blocks of a first size, and can perform hybrid transformation by synthesizing the blocks 302 of a first size quadrature-transformed by the quadrature transformer 201 and blocks of a first size of high-level images to generate a block of a second size.
The subband splitter 200, the quadrature transformer 201 and the block splitter 202 constitute a hybrid transformer 213.
A mode selector 203 compares a hybrid transformation coefficient 303 output from the block splitter 202 and a differential transformation coefficient 304 output from an adder 204, and changes over between a switch 215 and a switch 216.
The adder 204 calculates a difference between, the hybrid transformation coefficient 303 of an original image output from the block splitter 202 and a hybrid transformation coefficient 305 of a reference image output from a motion compensation predictor 205.
The motion compensation predictor 205 predicts a motion compensation of the hybrid transformation coefficient 303. A motion vector detector 206 detects a motion vector by comparing an output of a subband synthesizer 212 and an original image 300.
A quantizer 207 quantizes a signal from the switch 215 and outputs the same. An inverse quantizer 208 inverse-quantizes a signal 310 quantized by the quantizer 207.
An adder 209 adds and outputs an output signal 311 from the inverse quantizer 208 and a signal 307 from the switch 216.
A frame memory 210 rearranges the hybrid transformation coefficients 303 to generate a low-level image and a plurality of high-level images. An inverse quadrature transformer 211 splits the low-level image into blocks of a first size to conduct inverse quadrature transformation. The subband synthesizer 212 form the entire image through synthesis of subbands.
The frame memory 210, the inverse quadrature transformer 211 and the subband synthesizer 212 constitute a hybrid inverse transformer 214.
Operations of the aforementioned conventional art will now be described.
The subband splitter 200 subband-splits an original image into a low-level image 301 and a plurality of high-level images, and the quadrature transformer 201 splits the low-level image 301 into blocks of a first size to subject them to quadrature transformation. The block splitter 202 splits the plurality of high-level images further into blocks of the first size. It further synthesizes the quadrature-transformed blocks 302 of the first size and the blocks of the first size of the high-level images to form blocks of a second size, thereby performing a hybrid transformation. These are the operations in the hybrid transformer 213.
The frame memory 210 prepares a low-level image and a plurality of high-level images b rearranging the hybrid transformation coefficients 312. The inverse quadrature transformer 211 splits the low-level image into blocks of the first size and subjects the split blocks to an inverse quadrature transformation. The subband synthesizer 212 subband-synthesizes the entire image. These are the operations of the hybrid inverse transformer 214. As a result of these operations for hybrid inverse transformation, a decoded image 318 is generated. The resultant decoded image 318 is entered into the motion compensation predictor 205, where motion compensation prediction of the hybrid transformation coefficient 303 is conducted.
The adder 204 calculates a difference between the hybrid transformation coefficient 303 of the original image output from the block splitter 202 and the hybrid transformation coefficient 305 of the reference image output from the motion compensation predictor 205.
The mode selector 203 compares the hybrid transformation coefficient 303 output from the block splitter 202 and the differential transformation coefficient 304 output from the adder 204, selects any of them for coding for each block, and changes over the mode between the switch 215 and switch 216.
The quantizer 207 quantizes a signal from the switch 215, and the inverse quantizer 208 inverse-quantizes a quantized signal. The adder 209 adds an output signal 311 from the inverse quantizer 208 and a signal 307 from the switch 216, and outputs a restored hybrid transformation coefficient 312. The frame memory 210 restores stored hybrid transformation coefficients 312 while rearranging them in the form of low-level image (LL) and high-level images (LH, HL and HH). The inverse quadrature transformer 211 inverse-quadrature-transforms the low-level images (LL) of signals stored in the frame memory 210. The subband synthesizer 212 generates the reference image 318 by synthesizing an output 314 of the inverse quadrature transformer 211.
The aforementioned conventional case is typical conventional method comprising the steps of conducting subband-slitting first, and then quadrature transformation in the latter half. In this example, quadrature transformation is applied only to the low-level image (LL) of the original image obtained as result of subband splitting, and the image is reconstructed through rearrangement without applying quadrature transformation to the high-level images.
The above-mentioned method has a problem of image quality deterioration in the case of an image having may high-level component in the vertical or the horizontal direction.
The present invention was developed in view of the circumstances as described above, and has an object to provide high-efficiency image coding apparatus and image decoding apparatus which makes it possible to obtain a high-quality coded or decoded image less susceptible to block strain even at a high compression ratio.
The image coding apparatus and method of the present invention of splitting an image into a plurality of bands, then coding are based on a process comprising the steps of splitting an image into a plurality of bands by splitting means, applying quadrature transformation to images of the band components obtained by the splitting means, quantizing quadrature transformation coefficient obtained by the quadrature transforming means by the use of quantizing means, and variable-length-coding quantization coefficients of the individual band components obtained by the quantizing means, thereby generating a coded bit stream.
The image decoding apparatus and method of the present invention of splitting an image into a plurality of bands an based on a process comprising the steps of variable-length-decoding a coded bit stream of an image by a variable length decoding means, inverse-quantizing quantization coefficients of the plurality of band components obtained by the variable length decoding means, quadrature-inverse-transforming, by a quadrature inverse transforming means, quadrature transformation coefficients of the band components obtained by the inverse quantizing means, and synthesizing an image of the individual band components obtained by the quadrature inverse transforming means.